The present invention relates to variable displacement compressors that are employed in automobile air-conditioners.
A typical variable type compressor has a crank chamber housed in a housing and a rotatable drive shaft. The housing includes a cylinder block. Cylinder bores extend through the cylinder block about the drive shaft. A piston is accommodated in each cylinder bore. Each cylinder bore is connected to a discharge chamber through a discharge port. Refrigerant gas is compressed in each cylinder bore and discharged into the discharge chamber.
A pressurizing passage extends between the discharge chamber and the crank chamber. The compressed refrigerant gas in the discharge chamber is sent to the crank chamber through the pressurizing passage. The pressurizing passage has an inlet, which is opened to the discharge chamber, and an outlet, which is opened to the crank chamber. A discharge passage is also provided to return the refrigerant gas in the discharge chamber to an external refrigerant circuit.
A cam plate is fitted to the drive shaft in the crank chamber. The cam plate is supported in a manner such that it may incline while rotating integrally with the drive shaft. The peripheral portion of the cam plate is coupled to each piston. The inclination angle of the cam plate with respect to the axis of the drive shaft is altered to adjust the displacement of the compressor.
In this type of variable displacement compressor, the inlet of the pressurizing passage is located near the inlet of the discharge passage in the discharge chamber. Furthermore, the inlet of the discharge passage is located near the discharge port of each cylinder bore. Thus, when compressed refrigerant gas is discharged into the discharge chamber from the discharge port of each cylinder bore, some of the gas enters the discharge passage. This obstructs the flow of refrigerant gas from the pressurizing passage to the crank chamber.
When the compressor displacement is small, a large amount of hot pressurized refrigerant gas is sent to the crank chamber from the discharge chamber. However, it is difficult to continue sufficient lubrication of contacting parts in the crank chamber when the temperature and pressure in the crank chamber is high. Under such conditions, thermal expansion of mechanical components takes place and reduces the clearances provided between cooperating components. In addition, the viscosity of the lubricating oil suspended in the refrigerant gas may be decreased. As a result, the lubrication of the contacting parts may become insufficient.
This problem has been dealt with in various ways in the prior art. For example, the surface of the cam plate may be treated by thermal spraying a metal material such as copper to portions that contact other components. However, such treatment is costly and increases the weight of the cam plate. Furthermore, this increases the manufacturing cost and weight of the compressor.
Also, if the compressed refrigerant gas sent to the external refrigerant circuit includes a large amount of oil, a thick film of oil may form on the heat conducting surfaces of downstream devices, such as the condenser or the evaporator. This may reduce the heat exchanging efficiency of the heat exchanging devices and thus may reduce the refrigeration efficiency.